The present invention relates to the separation of 3,4-toluenediamine from a mixture containing 3,4- and 2,3-toluenediamine by selectively crystallizing from a solvent selected from the group consisting of toluene, xylene and benzene.
Toluene diisocyanate is a valuable intermediate in the preparation of polyurethanes. At present, about 500 million pounds are being consumed each year and it has been estimated that its use will increase to about 800 million pounds per year by the end of 1975.
Toluene diisocyanate is generally produced by dinitrating toluene to produce mixed dinitrotoluenes, reducing the mixed dinitrotoluenes to form a mixture of toluenediamines, separating from the desired 2,4- and 2,6-toluenediamine isomers a by-product fraction containing mixed o-toluenediamine isomers and converting the desired fraction to toluene diisocyanate by reaction with phosgene. The by-product fraction is generally separated by fractional distillation as disclosed in U.S. Pat. Nos. 3,149,162 and 3,637,514.
About 30 million pounds of the by-product fraction containing mixed o-toluenediamines is currently produced annually in the United States. A substantial amount of this is discarded. The remainder is put to uses where the mixed ortho isomers can be utilized without separation. An example of the use of the mixed ortho isomers is their conversion to mixed tolutriazoles for use as corrosion inhibitors in antifreezes and for use in other areas of metal treatment as disclosed in U.S. Pat. Nos. 3,637,514 and 3,732,239.
For many applications, it is desirable to separate 3,4-toluenediamine from the o-toluenediamine mixture. For example, it is known that the production of 6-methyl-1,3-dithiolo (4,5-6) quinoxalin-2-one, an insecticide, acaricide and fungicide requires substantially pure 3,4-toluenediamine as a starting material.
It is thus apparent that users of 3,4-toluenediamine are frequently unable to use the mixture of o-toluenediamine isomers which results as a by-product of the overall toluene diisocyanate process described above. Accordingly, much of the mixed ortho isomer mixture is discarded, destroyed or stockpiled due to the fact that an inexpensive process has not heretofore been available to separate out the useful 3,4- isomer which generally constitutes about 50% by weight of the o-toluenediamine mixture. It is, therefore, quite apparent that a substantial portion of the costs attributable to handling storing, shipping or discarding the o-toluenediamine mixture could be passed on to purchasers of the 3,4- isomer if an inexpensive means were available for separating the same from the o-toluenediamine mixture. Prior to the present invention, however, no such means was available.
Crystallization techniques are generally only used where it is desired to separate a pure component from a mixture containing the component and a minor amount of impurity (usually 5% of the mixture or less). Normally, however, where the impurity constitutes more than a minor amount of the mixture, one skilled in the art would not expect crystallization to be an effective tool to separate the desired component from the mixture. It would be expected that the use of crystallization techniques in such circumstances would not produce the desired degree of a separation. It was, therefore, quite surprising and unexpected to find that when an o-toluenediamine mixture containing 40 to 70% 3,4-toluenediamine and 30 to 60% 2,3-toluenediamine was dissolved in toluene and cooled, substantially pure 3,4-toluenediamine was obtained in high yields. It was also surprising in view of the structural similarity between the 2,3- and 3,4- isomers to find that crystallization could be carried out at widely varying temperatures to significantly increase yields without causing a corresponding decrease in product purity.
As used in this apecification and in appended claims, all percentages are by weight unless it is stated to the contrary.